Lubricant oil composition

ABSTRACT

Provided is a lubricating oil composition favorable for reducing friction in a sliding mechanism equipped with a piston ring and a liner, in a device having the sliding mechanism. The lubricating oil composition contains a lubricant base oil, (1) a polymethacrylate and/or an olefin copolymer having a mass-average molecular weight of 100,000 to 600,000, and (2) an ester-type ashless friction modifier and/or an amine-type ashless friction modifier, and has a shear viscosity at 150° C. of 2.3 mPa· or more and less than 3.7 mPa·s.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, inparticular to a lubricating oil composition favorable for reducingfriction in a device equipped with a piston ring and a liner, especiallyto a lubricating oil composition for internal combustion engines withimproved fuel-saving performance.

BACKGROUND ART

From the viewpoint of environmental load reduction, it is desired toreduce CO₂ to be emitted by automobiles for suppressing global warming,and it is desired to improve further fuel-saving performance oflubricating oil for internal combustion engines of automobiles, etc. Forimproving fuel-saving performance of lubricating oil for internalcombustion engines, improvement and viscosity reduction in point ofcomposition of lubricating oil are being under way in the region offluid lubrication (for example, PTLs 1 and 2). However, mere lubricatingoil viscosity reduction is problematic in point of lubricationinsufficiency (friction increase) in a severe lubrication environmentsuch as sliding between a piston ring and a liner, and therefore,optimal technique for formulating a lubricating oil is further desired.On the other hand, from the viewpoint of improvement of fuel efficiencyof engines, down-sized lightweight high-power engines are being popular,but also in this case, there still exist concerns about insufficientlubricity between a piston ring and a liner with increase in thermalload. PTL 3 tries to attain friction reduction and an excellentfuel-saving effect from the viewpoint of both materials of a piston ringof an engine and a lubricating oil, but even with such a lubricatingoil, the lubricity between a piston ring and a liner is stillinsufficient.

CITATION LIST Patent Literature

PTL 1: Japanese Patent 5044093

PTL 2: Japanese Patent 4643030

PTL 3: JP 2005-2888 A

SUMMARY OF INVENTION Technical Problem

As described above, from the viewpoint of improvement of fuel-savingperformance of a lubricating oil (engine oil) for internal combustionengines, investigations for viscosity reduction for reducing frictionresistance (viscosity reduction in a practical range) in the region offluid lubrication are being promoted. However, in lubrication forsliding between a piston ring and a liner, there exist both a fluidlubrication region and a boundary lubrication region as mixed, and underthe condition, mere viscosity reduction of an engine oil would result indominant boundary lubrication and there would be a risk of frictionresistance increase. Accordingly, a lubricating oil composition havingan optimal formulation capable of imparting excellent low frictioncharacteristics to sliding between a piston ring and a liner is desired.

Specifically, an object of the present invention is to provide alubricating oil composition favorable for friction reduction in asliding mechanism equipped with a piston ring and a liner, in a devicehaving the sliding mechanism.

Solution to Problem

As a result of assiduous studies made in consideration of theabove-mentioned problems, the present inventors have found that, when alubricating oil composition is controlled to have a shear viscosityfalling within a specific high-temperature shear viscosity range andcontrolled to contain a lubricant base oil, (1) a polymethacrylateand/or an olefin copolymer having a specific molecular weight and (2) aspecific friction modifier, the lubricating oil composition can reducefriction resistance in any of a fluid lubrication region and a mixedlubrication region to improve lubricity and, as a result, even when usedin a device having a sliding mechanism equipped with a piston ring and aliner, the lubricating oil composition can greatly reduce the frictionresistance against the thermal load increase in the device and canmaintain lubricity therein, and thus have completed the presentinvention.

Specifically, the present invention is as follows.

[1] A lubricating oil composition for use in a device having a slidingmechanism equipped with a piston ring and a liner, which contains alubricant base oil, (1) a polymethacrylate and/or an olefin copolymerhaving a mass-average molecular weight of 100,000 to 600,000, and (2) anester-type ashless friction modifier and/or an amine-type ashlessfriction modifier, and has a shear viscosity at 150° C. of 2.3 mPa·s ormore and less than 3.7 mPa·s.

[2] The lubricating oil composition according to the above [1], whereinthe mass-average molecular weight of the polymethacrylate and/or theolefin copolymer (1) is 200,000 to 550,000.

[3] The lubricating oil composition according to the above [1] or [2],wherein the content of the polymethacrylate and/or the olefin copolymer(1) is 2.5% by mass or more and less than 15% by mass based on the totalamount of the composition.

[4] The lubricating oil composition according to any one of the above[1] to [3], wherein the content of the ashless friction modifier is 0.1%by mass or more and less than 2% by mass based on the total amount ofthe composition.

[5] The lubricating oil composition according to any one of the above[1] to [4], wherein the viscosity index of the lubricant base oil is 120or more.

[6] The lubricating oil composition according to any one of the above[1] to [5], wherein the phosphorus content is 0.12% by mass or lessbased on the total amount of the composition.

[7] The lubricating oil composition according to any one of the above[1] to [6], which contains a Ca-containing metallic detergent and/or aMg-containing metallic detergent in an amount of 0.05% by mass or moreand 0.30% by mass or less as a total amount of Ca and Mg based on thetotal amount of the composition.

[8] The lubricating oil composition according to any one of the above[1] to [7], which contains polybutenylsuccinic imide and/or a boronatedpolybutenylsuccinic imide.

[9] The lubricating oil composition according to any one of the above[1] to [8], which is for internal combustion engines.

[10] The lubricating oil composition according to any one of the above[1] to [9], wherein the piston ring in a sliding mechanism equipped witha piston ring and a liner is one treated with chromium nitride.

[11] A method for producing the lubricating oil composition of any oneof the above [1] to [10], which includes a step of blending a lubricantbase oil with (1) a polymethacrylate and/or a olefin copolymer having amass-average molecular weight of 100,000 to 600,000 stated above and (2)an ester-type ashless friction modifier and/or an amine-type ashlessfriction modifier stated above.

[12] A method of lubricating a device having a sliding mechanismequipped with a piston ring and a liner, which includes lubricating adevice having a sliding mechanism equipped with a piston ring and aliner with the lubricating oil composition of any one of the above [1]to [10].

Advantageous Effects of Invention

According to the present invention, there can be provided a lubricatingoil composition favorable for reducing friction in a sliding mechanismequipped with a piston ring and a liner, in a device having the slidingmechanism.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic view showing an outline of a floating linerfriction tester for measuring the friction force between a piston ringand a liner.

DESCRIPTION OF EMBODIMENTS

The present invention is described in more detail hereinunder.

[Lubricating Oil Composition]

The lubricating oil composition of the present invention is alubricating oil composition for use in a device having a slidingmechanism equipped with a piston ring and a liner, which contains alubricant base oil, (1) a polymethacrylate and/or an olefin copolymerhaving a mass-average molecular weight of 100,000 to 600,000, and (2) anester-type ashless friction modifier and/or an amine-type ashlessfriction modifier, and has a shear viscosity at 150° C. of 2.3 mPa·s ormore and less than 3.7 mPa·s.

The lubricating oil composition of the present invention, when used in adevice having a sliding mechanism equipped with a piston ring and aliner, can reduce the friction resistance of the sliding mechanism.

Specifically, in general, in lubrication for sliding between a pistonring and a liner, in particular, the lubrication is likely to beperformed as a boundary lubrication region at around the top dead centerand the bottom dead center of the piston. Consequently, mere reductionin the viscosity of a lubricating oil composition for reducing thefriction in a fluid lubrication region causes dominant boundarylubrication at around the top dead center and the bottom dead center,therefore resulting in increase in the friction resistance therein. Inaddition, when the viscosity of a lubricating oil composition islowered, the friction resistance at a low temperature (around 30° C.)could be low but the friction resistance at a high temperature (around90° C.) may increase. On the other hand, when the viscosity of alubricating oil composition is increased, the friction resistance at ahigh temperature could be low but the friction resistance at a lowtemperature may increase. Consequently, it has been difficult to reducethe friction resistance from a low-temperature range to ahigh-temperature range.

Given the situation, the present inventors measured the friction energyof lubricating oil compositions prepared by blending various materials,using a floating liner friction tester to be mentioned below. As aresult, the present inventors have found that, when a polymethacrylateand an olefin copolymer are selected as materials for a lubricating oilcomposition and when the molecular weight of the materials is changed,the effect of the lubricating oil composition for reducing frictionenergy at a low temperature and/or a high temperature varies. Inaddition, the inventors have found that, when the kind of the frictionmodifier to be used along with the polymethacrylate and/or the olefincopolymer is changed, the effect of the lubricating oil composition forreducing friction energy at a low temperature and/or a high temperaturevaries.

More specifically, the inventors have found that, when a lubricating oilcomposition in which the molecular weight of the polymethacrylate is lowand oversteps the scope of the present invention is used, the frictionenergy at a low temperature (liner temperature of 30° C.) is high. Inturn, the inventors have also found that a lubricating oil compositioncontaining an ether-type friction modifier that is outside the scope ofthe present invention is used, the friction energy at a linertemperature of 90° C. is high. Further, the inventors have found that,in the case where the high-temperature shear viscosity (150° C.)oversteps the scope of the present invention, the friction energy at aliner temperature of 30° C. or 90° C. is high in any case.

The present invention has been created on the basis of these findings,and the lubricating oil composition contains, as described above, apolymethacrylate and/or an olefin copolymer whose molecular weight fallswithin a specific range, and a specific kind of a friction modifier, andhas a high-temperature shear viscosity (150° C.) falling within apredetermined range. Accordingly, using the lubricating oil compositionof the present invention, the friction resistance at a low temperature(around 30° C.) and a friction resistance at a high temperature (around90° C.) can be reduced. Consequently, when the lubricating oilcomposition is used in a device having a sliding mechanism equipped witha piston ring and a liner, the friction resistance can be reduced notonly in a fluid lubrication region but also in a boundary lubricationregion.

(Lubricant Base Oil)

The lubricant base oil for use in the lubricating oil composition of thepresent invention is not specifically limited, and any base oil composedof a mineral oil and/or a synthetic oil is usable. The kinematicviscosity of the base oil at 100° C. is preferably 7 mm²/s or less, morepreferably 6 mm²/s or less. When the kinematic viscosity at 100° C. is 7mm²/s or less, the fuel-saving performance can be realized withoutincreasing the friction coefficient in a fluid lubrication region. Onthe other hand, the kinematic viscosity at 100° C. is preferably 2 mm²/sor more, more preferably 3 mm²/s or more. When the kinematic viscosityat 100° C. is 2 mm²/s or more, lubricity, such as wear-resistantproperties and the like, necessary for slide portions such as valvetrain systems, pistons, rings, bearings and the like in internalcombustion engines can be secured.

Examples of mineral base oils include those refined by subjecting alubricating oil distillate that is obtained by distilling a crude oilunder atmospheric pressure or by distilling under reduced pressure theatmospheric residue given by atmospheric distillation of a crude oil, toone or more treatments selected from solvent deasphalting, solventextraction, hydro-cracking, solvent dewaxing, hydrorefining and thelike, and those produced by isomerization of a mineral oil wax or a waxproduced through Fischer-Tropsch synthesis or the like (gas-to-liquidwax).

These mineral base oils preferably have a viscosity index of 90 or more,more preferably 100 or more, even more preferably 120 or more. When theviscosity index is not lower than the above value, the low-temperatureviscosity of the composition can be reduced to realize fuel saving andthe high-temperature viscosity thereof can be increased to securelubricity at a high temperature. The viscosity index can be measuredaccording to JIS K 2283.

The aromatic content (% C_(A)) in the mineral base oil is preferably 3or less, more preferably 2 or less, even more preferably 1 or less. Thesulfur content is preferably 100 ppm by mass or less, more preferably 50ppm by mass or less. When the aromatic content is 3 or less and thesulfur content is 100 ppm by mass or less, the oxidation stability ofthe composition can be kept good.

On the other hand, examples of synthetic base oils include polybutene ora hydride thereof, poly-α-olefins, such as 1-decene oligomer, etc., orhydrides thereof, diesters such as di-2-ethylhexyl adipate,di-2-ethylhexyl sebacate, etc., polyol esters such as trimethylolpropanecaprylate, pentaerythritol 2-ethylhexanoate, etc., aromatic syntheticoils such as alkylbenzenes, alkylnaphthalenes, etc., polyalkyleneglycols, or mixtures thereof.

In the present invention, any of mineral base oils, synthetic base oils,or mixtures of any two or more selected from these can be used as thebase oil.

The content of the base oil in the lubricating oil composition of thepresent invention is preferably 60% by mass or more, more preferably 70%by mass or more, even more preferably 75% by mass or more, and ispreferably 90% by mass or less, more preferably 85% by mass or less,even more preferably 80% by mass or less.

((1) Polymethacrylate and/or Olefin Copolymer)

In the lubricating oil composition of the present invention, apolymethacrylate having a mass-average molecular weight of 100,000 to600,000 and/or an olefin copolymer having a mass-average molecularweight of 100,000 to 600,000 are blended especially for imparting anexcellent friction-reducing effect to a sliding mechanism equipped witha piston ring and a liner.

In the present invention, from the viewpoint of realizing an excellentfriction-reducing effect, a polymethacrylate having a mass-averagemolecular weight of 200,000 to 550,000 is preferably used. One alone ortwo or more such polymethacrylates may be used either singly or ascombined.

The mass-average molecular weight (Mw) can be measured, for example,according to the following method. Specifically, according to a gelpermeation chromatography (GPC) method and using an apparatus under thecondition mentioned below, a polystyrene-equivalent mass-averagemolecular weight of the polymer is measured, and the measured value canbe denoted as a mass-average molecular weight (Mw) thereof.

<GPC Apparatus>

Column: TOSO GMHHR-H(S)HT

Detector: RI detector for liquid chromatography, WATERS 150C

<Measurement Condition>

Solvent: 1,2,4-trichlorobenzene

Measurement temperature: 145° C.

Flow rate: 1.0 mL/min

Sample concentration: 2.2 mg/mL

Injection amount: 160

Calibration curve: Universal Calibration

Analysis program: HT-GPC (Ver. 1.0)

Examples of the olefin copolymer usable here include ethylene-propylenecopolymers, ethylene-butylene copolymers, styrene-isoprene copolymers,styrene-butadiene copolymers, etc.

The olefin copolymer may be used in combination with the above-mentionedpolymethacrylate.

The polymethacrylate and the olefin copolymer each have a mass-averagemolecular weight of 100,000 to 600,000. When the mass-average molecularweight is lower than 100,000, in particular, the friction-reducingeffect in a sliding mechanism equipped with a piston ring and a liner islow, but when it is more than 600,000, the friction-reducing effect on ahigh-temperature side is difficult to realize, and in any case, it isdifficult to stably maintain the effect. From this viewpoint, themass-average molecular weight of the polymethacrylate and the olefincopolymer is preferably 200,000 to 550,000 each, even more preferably220,000 to 520,000 each. The mass-average molecular weight can beobtained from a calibration curve formed through gel permeationchromatography using polystyrene.

The content of the polymethacrylate and the olefin copolymer ispreferably selected within a range of 2.5% by mass or more and less than15% by mass based on the total amount of the composition. When thecontent is 2.5% by mass or more, in particular, an excellentfriction-reducing effect in a sliding mechanism equipped with a pistonring and a liner can be realized, and when it is less than 15% by mass,an excellent friction-reducing effect can be realized with no problem ofviscosity increase at a low temperature, and in any case, the effect canbe maintained stably. From the above-mentioned viewpoint, the content ofthe polymethacrylate and the olefin copolymer (1) is more preferably3.5% by mass or more and 13.5% by mass or less based on the total amountof the composition.

((2) Friction Modifier)

The lubricating oil composition of the present invention contains anester-type ashless friction modifier and/or an amine-type ashlessfriction modifier especially for securing an excellent friction-reducingeffect in a sliding mechanism equipped with a piston ring and a liner.

As the ester-type ashless friction modifier and/or the amine-typeashless friction modifier, for example, an aliphatic ester or analiphatic amine having at least one of an alkyl group and an alkenylgroup having 6 to 30 carbon atoms in the molecule can be used. The alkylgroup and the alkenyl group each include those having a linear structureand those having a branched structure, and a linear alkyl group or alinear alkenyl group is preferred. The double bond in the alkenyl groupmay be at any arbitrary position.

Examples of the aliphatic ester having at least one alkyl group oralkenyl group having 6 to 30 carbon atoms in the molecule that ismentioned for the ester-type ashless friction modifier include esters ofa fatty acid having an alkyl group or an alkenyl group with 6 to 30carbon atoms and an aliphatic monoalcohol or an aliphatic polyalcohol,and specifically, preferred examples thereof include glycerinmonooleate, glycerin dioleate, sorbitan monooleate, sorbitan dioleate,etc. Containing glycerin monooleate is more preferred, and glycerinmonooleate is even more preferred. One alone or two or more differentkinds of the above-mentioned ester-type ashless friction modifiers maybe used either singly or as combined.

Examples of the aliphatic amine having at least one alkyl group oralkenyl group with 6 to 30 carbon atoms in the molecule that ismentioned for the amine-type ashless friction modifier include aliphaticmonoamines and alkylene oxide adducts thereof, alkanolamines, aliphaticpolyamines, imidazoline compounds, etc.

The aliphatic monoamine for use herein may be an aliphatic monoaminehaving 6 to 30 carbon atoms, preferably 12 to 24 carbon atoms, morepreferably 16 to 22 carbon atoms, and the aliphatic monoamine of thetype may be have linear structure or a branched structure, and may be asaturated one or an unsaturated one.

The alkylene oxide adduct of the aliphatic monoamine is preferably anadduct of an alkylene oxide having 2 or 3 carbon atoms and the aliphaticmonoamine. Specific examples of the alkylene oxide adduct of thealiphatic monoamine include various amine-type ashless frictionmodifiers, for example, monoethanolamine compounds such ashexylmonoethanolamine, heptylmonoethanolamine, octylmonoethanolamine,2-ethylhexylmonoethanolamine, nonylmonoethanolamine,decylmonoethanolamine, undecylmonoethanolamine, dodecylmonoethanolamine,tridecylmonoethanolamine, tetradecylmonoethanolamine,pentadecylmonoethanolamine, hexadecylmonoethanolamine,heptadecylmonoethanolamine, octadecylmonoethanolamine(stearylmonoethanolamine), 2-heptylundecylmonoethanolamine,nonadecylmonoethanolamine, eicosylmonoethanolamine,heneicosylmonoethanolamine, docosylmonoethanolamine,tricosylmonoethanolamine, tetracosylmonoethanolamine,11-ethyltricosylmonoethanolamine, pentacosylmonoethanolamine,hexacosylmonoethanolamine, heptacosylmonoethanolamine,octacosylmonoethanolamine, nonacosylmonoethanolamine,triacontylmonoethanolamine, etc.; diethanolamine compounds,monopropanolamine compounds and dipropanolamine compounds havingdiethanolamine, monopropanolamine or dipropanolamine, respectively, inplace of the monoethanolamine in the above-mentionedalkylmonoethanolamine compounds; octadecenylmonoethanolamine compoundsand octadecenyldiethanolamine compounds having an alkenyl group in placeof the alkyl group in the above-mentioned compounds, etc. Diethanolaminecompounds are more preferred.

In the present invention, among the above-mentioned amine-type ashlessfriction modifiers, at least one of octadecenyldiethanolamine andoctadecyldiethanolamine is preferably used, from the viewpoint of thefriction-reducing effect thereof.

One alone or two or more kinds of the above-mentioned amine-type ashlessfriction modifiers may be used either singly or as combined. In thepresent invention, the amine-type ashless friction modifier may be usedin combination with the above-mentioned ester-type ashless frictionmodifier.

The content of the ester-type ashless friction modifier and/or theamine-type ashless friction modifier in the present invention ispreferably 0.1% by mass or more and less than 2% by mass based on thetotal amount of the composition. When the content of the ashlessfriction modifier is 0.1% by mass or more, in particular, an excellentfriction-reducing effect in a sliding mechanism equipped with a pistonring and a liner, especially an excellent friction—modifying effect in amixed lubrication region can be favorably realized. In turn, even whenthe content is 2.0% by mass or more, any further improvement of theeffect worth the content increase could not be expected. From theabove-mentioned viewpoint, the content of the ester-type ashlessfriction modifier and/or the amine-type ashless friction modifier (2) ismore preferably 0.5% by mass or more and 1.5% by mass or less, even morepreferably 0.7% by mass or more and 1.3% by mass or less.

(Metallic Detergent)

Preferably, the lubricating oil composition of the present inventioncontains a metallic detergent. Examples of the metallic detergentinclude alkali metal (sodium (Na), potassium (K) or the like) oralkaline earth metal (calcium (Ca), magnesium (Mg), barium (Ba) or thelike) sulfonates, phenates, salicylates, naphthenates, etc. In thepresent invention, an alkaline earth metal, especially calcium (Ca)and/or magnesium (Mg)-containing metallic detergent is preferably usedas the metallic detergent, and sulfonates, phenates and salicylatesthereof are especially preferably used. One alone or two or moredifferent kinds of these may be used either singly or as combined.

The metallic detergent may be any of neutral salts, basic salts andoverbased salts. The total base number and the content of these metallicdetergents may be arbitrarily selected in accordance with the desiredperformance of the lubricating oil. The total base number is, accordingto a perchloric acid method, generally 500 mg-KOH/g or less, preferably20 mg-KOH/g or more and 400 mg-KOH/g or less. The content is generally0.1% by mass or more and 10% by mass or less based on the total amountof the lubricating oil composition, and is, as a total equivalent ofcalcium (Ca) and magnesium (Mg), 0.05% by mass or more and 0.3% by massor less, preferably 0.1% by mass or more and 0.3% by mass or less. Whenthe content of the metallic detergent is too small, the cleanlinesswould be insufficient; but when too much, the frictioncoefficient-reducing effect may be insufficient as the case may be.

The total base number as referred to herein means the total base numbermeasured through potentiometric titration (base number/perchloric acidmethod) according to 7. of JIS K 2501 “Petroleum Products andLubricating Oils—Test Method for Neutralization Number”.

(Polybutenylsuccinic Imide and/or a Boronated Polybutenylsuccinic Imide)

The lubricating oil composition of the present invention preferablycontains polybutenylsuccinic imide and/or a boronatedpolybutenylsuccinic imide as an ashless dispersant.

The polybutenylsuccinic imide has a polybutenyl group having anumber-average molecular weight of 900 to 3,500, and is generallyobtained by reacting a polybutenylsuccinic acid anhydride, which isobtained through reaction of a polybutene and a maleic anhydride, or analkylsuccinic acid anhydride obtained through hydrogenation thereof,with a polyamine.

The polyamine includes a simple diamine such as ethylenediamine,propylenediamine, butylenediamine, pentylenediamine, etc.; apolyalkylenepolyamine such as diethylenetriamine, triethylenetetramine,tetraethylenepentamine, pentaethylenehexamine,di(methylethylene)triamine, dibutylenetriamine, tributylenetetramine,pentapentylenehexamine, etc.; a piperazine derivative such asaminoethylpiperazine, etc.

In addition to the above-mentioned polybutenylsuccinic imide, theirborides and/or those prepared by modifying them with an organic acid arealso usable. The boronated polybutenylsuccinic imide for use herein maybe one produced according to an ordinary method. For example, apolybutenylsuccinic acid anhydride is prepared as above, and is thenreacted for imidation with an intermediate that is prepared by reactinga polyamine with a boron compound such as boron oxide, boron halide,boric acid, boric anhydride, borate ester, ammonium borate or the liketo prepare a boronated polybutenylsuccinic imide.

One alone or two or more different kinds of polybutenylsuccinic imidesand/or boronated polybutenylsuccinic imides may be used either singly oras combined.

The content of the polybutenylsuccinic imide and/or the boronatedpolybutenylsuccinic imide is 0.5% by mass or more and 15% by mass orless based on the total amount of the lubricating oil composition,preferably 1% by mass or more and 10% by mass or less. When the amountof the additive falls within the above range, the high-temperaturecleanability of the lubricating oil composition sufficiently improves,and the low-temperature flowability thereof also significantly improves.The content of the polybutenylsuccinic imide and/or the boronatedpolybutenylsuccinic imide is preferably 0.04% by mass or more and 40% bymass or less as a succinimide compound-derived nitrogen content based onthe total amount of the lubricating oil composition. Further, in thecase where the succinimide compound contains a boride thereof, the boroncontent derived from the boride is preferably 0.01% by mass or more and0.3% by mass or less based on the total amount of the composition. Whenthe boron content falls within the range, good cleanability anddispersibility can be realized.

(Other Additives)

The lubricating oil composition of the present invention may furthercontain, as blended therein, an anti-wear agent, an extreme-pressureagent, an antioxidant, a friction modifier, a pour point depressant, arust inhibitor, a deactivator, a defoaming agent, etc., in addition tothe above-mentioned various additives. Further, in addition to thepolymethacrylate or the olefin copolymer having a specific molecularweight and the specific friction modifier in the present invention, thelubricating oil composition may optionally contain any other viscosityindex improver, friction modifier, etc.

The anti-wear agent and the extreme-pressure agent may be suitablyselected from any known anti-wear agent and extreme-pressure agent thatare heretofore used as an anti-wear agent and an extreme-pressure agentin engine oils. For example, there are mentioned metal (Zn, Pb, Sb, Mo,etc.) dithiophosphates, metal (Zn, Pb, Sb, Mo, etc.) dithiocarbamates,metal (Pb, etc.) naphthenates, metal (Pb, etc.) salts of fatty acids,boron compounds, phosphate esters, phosphite esters, alkylhydrogenphosphites, phosphate amine salts, phosphate metal salt (Zn, etc.),disulfides, sulfurized oils and fats, sulfurized olefins, dialkylpolysulfides, diarylalkyl polysulfides, diaryl polysulfides, etc. Onealone or two or more kinds of these anti-wear agents andextreme-pressure agents may be used either singly or as combined, and ingeneral, the content thereof falls within a range of 0.1% by mass ormore and 5% by mass or less based on the total amount of the lubricatingoil composition.

The antioxidant for use herein may be suitably selected from any knownantioxidants heretofore generally used as an antioxidant in engine oils.Phenolic antioxidants, amine-type antioxidants, molybdenum-containingantioxidants, sulfur-containing antioxidants, phosphorus-containingantioxidants and the like are preferably used. Concretely, there arementioned amine-type antioxidants such as alkylated diphenylamines,phenyl-α-naphthylamines, alkylated phenyl-α-naphthylamines, etc.;phenolic antioxidants such as 2,6-di-tert-butylphenol,-methylenebis(2,6-di-tert-butylphenol),isooctyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, octadecyldi-tert-butyl-4-hydroxyphenyl)propionate, etc.; sulfur-containingantioxidants such as dilauryl-3,3′-thiodipropionate, etc.;phosphorus-containing antioxidants such as phosphite, etc.; and furthermolybdenum-containing antioxidants. One or more kinds of theseantioxidants may be used either singly or as combined, but in general,using two or more is preferred. The content is preferably 0.01% by massor more and 5% by mass or less based on the total amount of thelubricating oil composition, more preferably 0.2% by mass or more and 3%by mass or less.

Examples of the friction modifier include organic molybdenum compounds,fatty acids, higher alcohols, oils and fats, amides, sulfurized esters,phosphate esters, phosphite esters, phosphate amine salts, etc. Onealone or two or more different kinds of these friction modifiers may beused either singly or as combined in any desired manner, and in general,the content thereof falls within a range of 0.05% by mass or more and 4%by mass or less based on the total amount of the lubricating oilcomposition.

Examples of the pour point depressant include ethylene/vinyl acetatecopolymers, condensation products of paraffin chloride and naphthalene,condensation products of paraffin chloride and phenol,polymethacrylates, polyalkylstyrenes, etc. The content of the agentgenerally falls within a range of 0.01% by mass or more and 5% by massor less based on the total amount of the lubricating oil composition.

Examples of the rust inhibitor include fatty acids, alkenylsuccinic acidhalf esters, fatty acid soaps, alkylsulfonate salts, fatty acid amines,paraffin oxides, alkylpolyoxyethylene ethers, etc., and in general, thecontent thereof is within a range of 0.01% by mass or more and 3% bymass or less based on the lubricating oil composition.

The metal deactivator includes benzotriazole, triazole derivatives,benzotriazole derivatives, thiadiazole derivatives, etc., and ingeneral, the content thereof is within a range of 0.01% by mass or moreand 3% by mass or less based on the total amount of the lubricating oilcomposition.

Examples of the defoaming agent include dimethylpolysiloxanes,polyacrylates, etc.

(Lubricating Oil Composition)

The lubricating oil composition of the present invention contains theabove-mentioned lubricant base oil, the above-mentioned indispensablecomponents, and optionally the above-mentioned various additives.

In the lubricating oil composition of the present invention, preferably,the phosphorus content is 0.12% by mass or less based on the totalamount of the lubricating oil composition. In general, it is oftenpreferable that the phosphorus content in the composition is large insome degree from the viewpoint of wear-resistant properties, etc., buton the other hand, reducing the content of a phosphorus-containingcompound is desired from the viewpoint of environmental load reduction.In the present invention, even when the phosphorus content is low, thatis, 0.12% by mass or less, the composition can exhibit an excellentfriction-reducing effect. From this viewpoint, the phosphorus content ismore preferably 0.10% by mass or less based on the total amount of thelubricating oil composition.

The phosphorus content may be controlled to be the content of thephosphorus-containing additive mentioned above. For example, typicalphosphorus-containing anti-wear agents include phosphate esters,thiophosphate esters, especially zinc dithiophosphate (ZnDTP), and useand the content of these additives may be suitably controlled.

The lubricating oil composition of the present invention has a shearviscosity at 150° C. of 2.3 mPa·s or more and 3.7 mPa·s or less. Whenthe shear viscosity at 150° C. is lower than 2.3 mPa·s, thefriction-reducing effect on a high-temperature side is not sufficient,but when the shear viscosity is higher than 3.7 mPa·s, thefriction-reducing effect on a low-temperature side is insufficient onthe contrary. From the viewpoint, the shear viscosity of the lubricatingoil composition of the present invention at 150° C. is preferably 2.5mPa·s or more and 3.5 mPa·s or less.

The “shear viscosity at 150° C.” in the present invention can becontrolled, for example, by controlling the molecular weight and thecontent of the polymethacrylate and/or the olefin copolymer (1) andother viscosity index improver, etc., and by controlling the viscosityand the like of the base oil. Regarding the measurement method, theviscosity of the composition is measured after shorn at a shearing rateof 10⁶/s at 150° C., according to JPI-5S-36-2003.

The kinematic viscosity of the lubricating oil composition of thepresent invention at 40° C. is preferably 20 mm²/s to 100 mm²/s, morepreferably 30 mm²/s to 80 mm²/s, more preferably 40 mm²/s to 70 mm²/s.Also preferably, the kinematic viscosity at 100° C. is 5 mm²/s to 30mm²/s, more preferably 5 mm²/s to 20 mm²/s, more preferably 6 mm²/s to15 mm²/s. When the kinematic viscosity at 40° C. or 100° C. falls withinthe above range, an excellent friction-reducing effect can be favorablyrealized.

The viscosity index of the lubricating oil composition of the presentinvention is preferably 120 or more. When the viscosity index is 120 ormore, the low-temperature viscosity of the composition can be low torealize fuel saving and the high-temperature viscosity thereof can behigh to secure lubricity at a high temperature. From this viewpoint, theviscosity index of the lubricating oil composition of the presentinvention is preferably 140 or more, more preferably 160 or more, evenmore preferably 180 or more, still more preferably 200 or more. Thekinematic viscosity and the viscosity index can be measured according toJIS K 2283.

(Friction Energy of Lubricating Oil Composition)

In the present invention, the friction energy of the lubricating oilcomposition can be measured using a floating liner friction tester shownin FIG. 1. The floating liner friction tester shown in FIG. 1 isdescribed below.

The floating liner friction tester 1 has a block 2 that has a pistonmovement pathway 2 a and crankshaft housing part 2 b, a liner 12arranged along the inner wall of the piston movement pathway 2 a, apiston 4 housed in the liner 12, piston rings 6 and 8 outwardly fittedto the piston 4, a crankshaft 10 housed in the crankshaft housing part 2b, a connecting rod 9 connecting the crankshaft 10 and the piston 4, anda load monitoring sensor 14 sandwiched between the liner 12 and thepiston movement pathway 2 a to monitor the friction force given betweenthe piston rings 6 and 8 and the liner 12 by the piston reciprocatingmotion of the piston 4.

The crankshaft 10 is rotationally driven by a motor (not shown) toinduce the reciprocating motion of the piston 4 via the connecting rod9.

The load monitoring sensor 14 is fixed to the liner 12 via a fixationscrew 18. The floating liner friction tester 1 may be provided with athermometer 16 for measuring the temperature of the liner 12, as shownin FIG. 1.

In the floating liner friction tester 1, the friction force givenbetween the piston ring 6 and the liner 12 by the movement of the piston4 is measured by the load monitoring sensor 14.

In the floating liner friction tester 1 having the constitution asabove, a lubricating oil composition 20 is filled in the crankshafthousing part 2 b so that the liquid level could be higher than thecenter of the center axis of the crankshaft 10 and lower than the topend of the center axis. The lubricating oil composition 20 in thecrankshaft housing part 2 b is fed between the liner 12 and the pistonring 6 by the splash motion of the rotating crankshaft 10.

The friction energy of the lubricating oil composition of the presentinvention at a liner temperature of 90° C., as measured using thefloating liner friction tester 1 having the specifications mentionedbelow under the following measurement conditions, is preferably 4.6J/rotation or less, more preferably 4.4 J/rotation or less, from theviewpoint of reducing the friction in a sliding mechanism.

<Specifications of Floating Liner Friction Tester 1>

Test apparatus: floating liner friction tester driven by electromotor

Displacement: 315 cm³ (single cylinder)

Ring material: steel (CrN coating for surface treatment)

Liner material: FC250 cast iron

<Measurement Conditions for Floating Liner Friction Tester 1>

Liner temperature: 90° C.

Number of rotation: 900 rpm

Measurement item: friction force given to liner part (unit: N)

Evaluation item: friction energy per one rotation calculated fromfriction force (unit: J/rotation)

The friction energy of the lubricating oil composition of the presentinvention, as measured using the floating liner friction tester 1 havingthe specifications mentioned above and under the same conditions asabove except that the liner temperature is changed to 30° C., ispreferably 4.3 J/rotation or less, more preferably 4.0 J/rotation orless, even more preferably 3.5 J/rotation or less, from the viewpoint ofreducing the friction in a sliding mechanism.

(Production Method for Lubricating Oil Composition)

The lubricating oil composition of the present invention may be producedaccording to a production method including a step of blending theabove-mentioned lubricant base oil with the above-mentioned (1)polymethacrylate and/or olefin copolymer having a mass-average molecularweight of 100,000 to 600,000 and the above-mentioned (2) ester-typeashless friction modifier and/or amine-type ashless friction modifier.

The details of the indispensable components are as mentioned above.Along with the indispensable components, the above-mentioned optionalcomponents may also be blended. Further, the production method of thepresent invention may include any other step than the above-mentionedstep.

(Use in Device Having Sliding Mechanism Equipped with Piston Ring andLiner)

The lubricating oil composition of the present invention is, as havingthe above-mentioned effects and advantages, suitable for lubrication ofa sliding mechanism equipped with a piston ring and a liner in a devicehaving such a sliding mechanism equipped with a piston ring and a liner,especially for lubrication of a sliding mechanism equipped with a pistonring and a liner of an internal combustion engine.

The material of the piston ring and a cylinder liner to which thelubricating oil composition of the present invention is applied is notspecifically limited. In general, not only aluminum but also cast ironalloys are employable as the material for a cylinder liner, and as thematerial for a piston ring, an Si—Cr steel or a martensite stainlesssteel with 11 to 17 mass % Cr is usable. The piston ring is preferablysubjected to surface treatment of chromium plating treatment, chromiumnitride treatment, nitriding treatment or a combination of any of thesetreatments. In the present invention, from the viewpoint of realizingexcellent friction reduction, adhesiveness and durability, use of thelubricating oil composition of the present invention in a slidingmechanism equipped with a piston ring and a liner where the piston ringis processed through chromium nitridation treatment is preferred ascapable of further increasing the advantageous effects of the presentinvention.

From the viewpoint of further enhancing fuel-saving performance, thepresent invention is favorably applied to a sliding mechanism equippedwith a piston ring and a liner of an internal combustion engine ofautomobiles.

[Lubrication Method for Device Having Sliding Mechanism Equipped withPiston Ring and Liner]

The present invention also relates to a lubrication method oflubricating a device having a sliding mechanism equipped with a pistonring and a liner, using the lubricating oil composition of the presentinvention. Specifically, the present invention relates to a lubricationmethod for a device having a sliding mechanism equipped with a pistonring and a liner, wherein a device having a sliding mechanism equippedwith a piston ring and a liner is lubricated with a lubricating oilcomposition that contains a lubricant base oil, (1) a polymethacrylateand/or an olefin copolymer having a mass-average molecular weight of100,000 to 600,000, and (2) an ester-type ashless friction modifierand/or an amine-type ashless friction modifier, and has a shearviscosity at 150° C. of 2.3 mPa·s or more and less than 3.7 mPa·s. Thelubricating oil composition and the sliding mechanism equipped with apiston ring and a liner in the present invention are as described above.

In the present invention, the lubricating oil composition of the presentinvention is used as a lubricating oil in a sliding part between apiston ring and a cylinder liner to greatly reduce the friction thereinin any condition of fluid lubrication or mixed lubrication, therebycontributing toward improvement of fuel-saving performance in the part.

EXAMPLES

Next, the present invention is described concretely with reference toExamples, but the present invention is not whatsoever restricted bythese Examples.

[Evaluation Items, Evaluation Methods]

The properties of the lubricating oil were determined according to thefollowing methods.

(1) Kinematic viscosity (40° C., 100° C.): According to JIS K 2283.(2) Viscosity index: According to JIS K 2283.(3) Base number: Potentiometric titration (base number/perchloric acidmethod) according to 7. of JIS K 2501 “Petroleum Products andLubricating Oils—Test Method for Neutralization Number”.(4) Phosphorus content: According to JPI-5S-38-92.(5) Shear viscosity: According to JPI-5S-36-2003, the viscosity wasmeasured after shorn at a shearing rate of 10⁶/s at 150° C.(6) Friction amount and friction energy: Using a floating liner frictiontester shown in FIG. 1, each lubricant composition was tested for thefriction force between the piston ring and the liner under the conditionmentioned below, from which the friction energy per one rotation (unit:J/rotation) was calculated.

Test apparatus: floating liner friction tester driven by electromotor(FIG. 1)

Displacement: 315 cm³ (single cylinder)

Ring material: steel (CrN coating for surface treatment)

Liner material: FC250 cast iron

Test condition:

Liner temperature: 30° C. and 90° C.

Number of rotation: 900 rpm

Measurement item: friction force given to liner part (unit: N)

Evaluation item: friction energy per one rotation calculated fromfriction force (unit: J/rotation)

Examples 1 to 6 and Comparative Examples 1 to 7

As shown in Table 1, various additives were added to the base oil shownin the same Table to prepare a lubricating oil composition. Theresultant lubricating oil composition was tested to measure theproperties thereof such as the shear viscosity (150° C.), the kinematicviscosity (40° C., 100° C.), the viscosity index, etc., and the frictionenergy in the floating liner friction test was evaluated. The resultsare shown in Table 1.

TABLE 1 Example 1 2 3 4 5 6 Lubricating oil Base oil Hydrorefined baseoil 70N Composition (mass %) Hydrorefined base oil 100N balance balancebalance balance balance balance Hydrorefined base oil 150N Hydrorefinedbase oil 500N Additives PMA1 Mw. 400,000 13.00 PMA2 Mw. 230,000 8.006.20 PMA3 Mw. 45,000 OCP Mw. 500,000 8.30 8.30 4.00 Zincdialkyldithiophosphate A 0.20 0.20 0.20 0.20 0.20 0.20 Zincdialkyldithiophosphate B 1.20 1.20 1.20 1.20 1.20 1.20 Antioxidant A0.50 0.50 0.50 0.50 0.50 0.50 Antioxidant B 0.50 0.50 0.50 0.50 0.500.50 Metallic detergent A 1.20 1.20 1.20 1.20 1.20 1.20 Metallicdetergent B 1.00 1.00 1.00 1.00 1.00 1.00 Polybutenylsuccinbisimide 4.004.00 4.00 4.00 4.00 4.00 Boronated 1.00 1.00 1.00 1.00 1.00 1.00polybutenylsuccinmonoimide Amine-type friction modifier 1.00 1.00 1.001.00 1.00 Ester-type friction modifier 1.00 Ether-type friction modifierOther additive 0.80 0.80 0.80 0.80 0.80 0.80 Properties and Phosphoruscontent [mass %] 0.11 0.11 0.11 0.11 0.11 0.11 Performance Kinematic 40°C. [mm²/s] 47.3 48.6 47.1 65.1 65.1 44.2 of Lubricating viscosity 100°C. [mm²/s] 11.0 10.7 10.1 11.7 11.7 8.35 oil Viscosity index [—] 234 218209 177 177 168 Composition Shear viscosity (150° C.) [mPa · s] 3.4 3.43.1 3.4 3.4 2.7 Friction energy at liner temperature 90° C. 4.6 4.3 4.44.3 4.3 4.4 [J/rotation] Friction energy at liner temperature 30° C. 3.44.1 3.9 4.1 4.1 4.0 [J/rotation] Comparative Example 1 2 3 4 5 6 7Lubricating oil Base oil Hydrorefined base oil 70N 40.00 CompositionHydrorefined base oil 100N balance balance balance balance balancebalance (mass %) Hydrorefined base oil 150N 44.00 Hydrorefined base oil500N balance Additives PMA1 Mw. 400,000 18.00 2.10 13.00 13.00 PMA2 Mw.230,000 PMA3 Mw. 45,000 8.10 OCP Mw. 500,000 Zinc dialkyldithiophosphateA 0.20 0.20 0.20 0.20 0.20 0.20 0.20 Zinc dialkyldithiophosphate B 1.201.20 1.20 1.20 1.20 1.20 1.20 Antioxidant A 0.50 0.50 0.50 0.50 0.500.50 0.50 Antioxidant B 0.50 0.50 0.50 0.50 0.50 0.50 0.50 Metallicdetergent A 1.20 1.20 1.20 1.20 1.20 1.20 1.20 Metallic detergent B 1.001.00 1.00 1.00 1.00 1.00 1.00 Polybutenylsuccinbisimide 4.00 4.00 4.004.00 4.00 4.00 4.00 Boronated 1.00 1.00 1.00 1.00 1.00 1.00 1.00polybutenylsuccinmonoimide Amine-type friction modifier 1.00 1.00 1.001.00 1.00 Ester-type friction modifier Ether-type friction modifier 1.00Other additive 0.80 0.80 0.80 0.80 0.80 0.80 0.80 Properties andPhosphorus content [mass %] 0.11 0.11 0.11 0.11 0.11 0.11 0.11Performance Kinematic 40° C. [mm²/s] 86.6 26.2 46.0 55.7 29.8 47.4 47.3of Lubricating viscosity 100° C. [mm²/s] 11.1 5.39 9.20 13.3 5.95 11.111.0 oil Viscosity index [—] 115 146 187 248 153 235 234 CompositionShear viscosity (150° C.) [mPa · s] 3.4 2.0 3.4 4.0 2.2 3.4 3.4 Frictionenergy at liner temperature 90° C. 4.2 5.0 4.6 4.0 4.9 5.6 5.5[J/rotation] Friction energy at liner temperature 30° C. 5.8 4.0 4.4 5.03.0 3.4 3.4 [J/rotation]

The base oils and the additives used are as follows.

(1) Hydrorefined Base Oil

70N: kinematic viscosity at 40° C., 12.5 mm²/s; kinematic viscosity at100° C., 3.1 mm²/s; viscosity index, 109; % CA, 0.0; sulfur content,less than 10 mass ppm.

100N: kinematic viscosity at 40° C., 19.6 mm²/s; kinematic viscosity at100° C., 4.2 mm²/s; viscosity index, 122; % CA, 0.0; sulfur content,less than 10 mass ppm.

150N: kinematic viscosity at 40° C., 31.0 mm²/s; kinematic viscosity at100° C., 5.35 mm²/s; viscosity index, 105; % CA, 0.0; sulfur content,less than 10 mass ppm.

500N: kinematic viscosity at 40° C., 90.5 mm²/s; kinematic viscosity at100° C., 10.9 mm²/s; viscosity index, 107; % CA, 0.0; sulfur content,less than 10 mass ppm.

(2) PMA1: polymethacrylate (mass-average molecular weight, 400,000)(3) PMA2: polymethacrylate (mass-average molecular weight, 230,000)(4) PMA3: polymethacrylate (mass-average molecular weight, 45,000)(5) OCP: olefin copolymer (mass-average molecular weight, 500,000)(6) Zinc dialkyldithiophosphate A: Zn content, 8.9 mass %; phosphoruscontent, 7.4 mass %, primary alkyl-type zinc dialkyldithiophosphate(7) Zinc dialkyldithiophosphate B: Zn content, 9.0 mass %; phosphoruscontent, 8.2 mass %, secondary alkyl-type zinc dialkyldithiophosphate(8) Antioxidant A: amine-type antioxidant(9) Antioxidant B: phenolic antioxidant(10) Metallic detergent A: overbased calcium salicylate [base number(perchloric acid method) 350 mg KOH/g, calcium content 12.1 mass %](11) Metallic detergent B: overbased calcium salicylate [base number(perchloric acid method) 225 mg KOH/g, calcium content 7.8 mass %](12) Polybutenylsuccinic bisimide: number-average molecular weight ofpolybutenyl group, 2000; base number (perchloric acid method), 11.9 mgKOH/g; nitrogen content, 0.99 mass %(13) Boronated polybutenylsuccinic monoimide: number-average molecularweight of polybutenyl group, 1000; base number (perchloric acid method),25 mg KOH/g; nitrogen content, 1.23 mass %; boron content, 1.3 mass %(14) Amine-type friction modifier: octadecyldiethanolamine(15) Ester-type friction modifier: glycerin monooleate(16) Ether-type friction modifier: polyglycerin Monooleyl ether(17) Other additives: pour point depressant, rust inhibitor, defoamingagent, etc.

The compositions of Examples 1 to 6 that are lubricating oilcompositions of the present invention are ones produced by adding anamine-type friction modifier or an ester-type friction modifier to anoil prepared by incorporating a polymethacrylate or an olefin copolymer,whose molecular weight falls within the scope defined in the presentinvention, in a base oil, and the high-temperature shear viscosity (150°C.) thereof is controlled to fall within the scope defined in thepresent invention. In the floating liner friction test, the frictionenergy of each of these compositions was low under both conditions of aliner temperature 30° C. and a liner temperature 90° C.

On the other hand, in Comparative Examples 1 and 2, the polymethacrylateand the olefin copolymer were not blended. The friction energy with thecomposition of Comparative Example 1 was high at a liner temperature 30°C. The viscosity of the composition of Comparative Example 2 was toolow, and therefore the friction energy with the composition at a linertemperature 90° C. was high. With the composition of Comparative Example3 where the molecular weight of the polymethacrylate is low andoversteps the scope in the present invention, the friction energy at aliner temperature 30° C. was high. In Comparative Examples 4 and 5, thehigh-temperature shear viscosity (150° C.) oversteps the scope definedin the present invention, and therefore, the friction energy at a linertemperature 30° C. and 90° C. was high. In Comparative Examples 6 and 7,the friction modifier in the present invention was not blended, or theether-type friction modifier not for use in the present invention wasblended, and therefore the friction energy at a liner temperature 90° C.was high.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention greatly reducesthe friction in a sliding mechanism equipped with a piston ring and aliner, and contributes toward environmental load reduction andimprovement of fuel-saving performance, and therefore favorably used asa lubricating oil for devices having a sliding mechanism equipped with apiston ring and a liner, especially for internal combustion engines.

REFERENCE SIGNS LIST

-   1: Floating Liner Friction Tester-   2: Block-   2 a: Piston Movement Pathway-   2 b: Crankshaft Housing Part-   4: Piston-   6, 8: Piston Ring-   10: Crankshaft-   12: Liner-   14: Load Monitoring Sensor-   16: Thermometer

1. A lubricating oil composition for use in a device having a slidingmechanism equipped with a piston ring and a liner, which comprises alubricant base oil, (1) a polymethacrylate and/or an olefin copolymerhaving a mass-average molecular weight of 100,000 to 600,000, and (2) anester-type ashless friction modifier and/or an amine-type ashlessfriction modifier, and has a shear viscosity at 150° C. of 2.3 mPa·s ormore and less than 3.7 mPa·s.
 2. The lubricating oil compositionaccording to claim 1, wherein the mass-average molecular weight of thepolymethacrylate and/or the olefin copolymer (1) is 200,000 to 550,000.3. The lubricating oil composition according to claim 1, wherein thecontent of the polymethacrylate and/or the olefin copolymer (1) is 2.5%by mass or more and less than 15% by mass based on the total amount ofthe composition.
 4. The lubricating oil composition according to claim1, wherein the content of the ashless friction modifier is 0.1% by massor more and less than 2% by mass based on the total amount of thecomposition.
 5. The lubricating oil composition according to claim 1,wherein the viscosity index of the lubricant base oil is 120 or more. 6.The lubricating oil composition according to claim 1, wherein thephosphorus content is 0.12% by mass or less based on the total amount ofthe composition.
 7. The lubricating oil composition according to claim1, which contains a Ca-containing metallic detergent and/or aMg-containing metallic detergent in an amount of 0.05% by mass or moreand 0.30% by mass or less as a total amount of Ca and Mg based on thetotal amount of the composition.
 8. The lubricating oil compositionaccording to claim 1, which comprises polybutenylsuccinic imide and/or aboronated polybutenylsuccinic imide.
 9. The lubricating oil compositionaccording to claim 1, which is for internal combustion engines.
 10. Thelubricating oil composition according to claim 1, wherein the pistonring in a sliding mechanism equipped with a piston ring and a liner isone treated with chromium nitride.
 11. A method for producing thelubricating oil composition of claim 1, which comprises a step ofblending a lubricant base oil with (1) a polymethacrylate and/or aolefin copolymer having a mass-average molecular weight of 100,000 to600,000 stated above and (2) an ester-type ashless friction modifierand/or an amine-type ashless friction modifier stated above.
 12. Amethod of lubricating a device having a sliding mechanism equipped witha piston ring and a liner, which comprises lubricating a device having asliding mechanism equipped with a piston ring and a liner with thelubricating oil composition of claim 1.